The present invention generally relates to a method for fastening a printed board to an element and a guide element used in the method.
The assembly process for an apparatus that comprises a printed board assembly involves several automatic assembly steps and often also at least some manual assembly steps. One of the assembly steps is to surface mount electronic components on a printed board. This is often done by applying a soldering paste on the printed board and automatically place the electronic components on the printed board in a so-called SMD-line by a so-called pick and place machine. After that, the electronic components are fixed to the printed board by soldering, for example in a soldering furnace. For the purpose of this patent application, the term xe2x80x98electronic componentxe2x80x99 is used for various electric or electronic components such as capacitors, resistors, transistors and integrated circuit chips which could be mounted on a printed board to create a printed board assembly.
Another step in the assembly process is to fasten the printed board assembly to an element, such as a chassis, base plate or the like. For this purpose, the printed board and the element are provided with to each other corresponding holes for reception of a screw that secures a proper fastening of the printed board and the element to each other. The screwing action is often done by an automatic screw machine, which fasten screws at predetermined positions corresponding to the positions of the holes in the printed board and the element when the printed board assembly has been guided to a position on the element. The position of the printed board at this stage is very important since the automatic screw machine typically demands a tolerance zone of xc2x10.1 mm for the positions of the holes in the printed board and the element, in a horizontal plane in relation to an outer frame of reference, to be able to centre the screw in the hole and engage in corresponding threads in the hole. If the position of a hole deviates more than 0.1 mm, the screw may not engage in a corresponding receiving thread and it may also damage the printed board assembly and/or the element as it is forced downwards by the screw machine.
To ensure a sufficiently accurate positioning of the printed board assembly on the element, separate guide pins are pressed down in another set of holes in the element. When the printed board assembly is guided towards the element, these guide pins guide the printed board assembly with the help of a set of corresponding guide holes in the printed board. The insertion of a guide pin in one of the corresponding holes in the element is often done manually and the guide pin is secured to the hole by a press fit, which demands very small limits of tolerance for the contacting surfaces of the guide pin and the wall of the hole in the element. Moreover, the element, provided that it is e.g. a chassis, is often surface covered with a coating material in order to improve the electric conductibility between the element and the printed board assembly. This coating material often create bulges around the edges between the holes for the guide pins and the surface, as well as bulges inside the holes. In order to allow proper press fit, these bulges need to be smoothed. Therefore, small limits of tolerance for the guide pins and the corresponding holes in the element generate higher fabrication costs than guide pins and holes with larger limits of tolerance. Also, the manual insertion makes great demands upon a worker when inserting a guide pin, since the press fit easily gives a slight inclination from the desired guide direction of the guide pin. The inclination might cause inaccurate positioning of the printed board assembly. Of course the same demands are true for an automatic guide pin inserting machine, if it is to replace the worker.
Alternatively, but less common, the element is provided with integrated guides for the guide holes in the printed board. Thereby the step of inserting a guide pin in a corresponding hole is avoided, but causes other disadvantages. The element, to which the printed board assembly is to be fastened, is often made of metal in order to conduct heat away from a printed board assembly in use and to improve the earthing. Such metal elements, such as chassis, are fabricated in advanced pre-programmed automatic machines, such as a CNC-machine, which is expensive to use and maintain. Small details, such as protruding integrated guides on a flat surface, give a much longer, in many cases unacceptable, fabrication time in the CNC-machine. Also, larger pieces of raw material have to be used.
Although the two above alternative procedures relates to a fastening of a printed board assembly, the same problems and disadvantages of the procedures are also true for a printed board.
In EP-376659-A1 a similar problem with separate guide pins is discussed in the field of multicontact connectors that use a base frame in which guide pins are press-fitted for guiding a large scale integrated circuit package. The guide pins are individually received in reference holes formed through a printed circuit board, whereby the base frame is positioned relative to the printed circuit board.
It is therefore a general object of the present invention to provide a method and a guide element for fastening a printed board or a printed board assembly in order to solve the above mentioned accuracy problems and disadvantages.
Another general object is to provide an improved method and a guide element that makes the fastening more effective in terms of cost and assembly time.
The invention therefore provide a method for fastening a printed board to an element, where the method comprises the steps of: mounting at least one guide element to the printed board before the printed board is guided towards the element, fastening the at least one guide element to the printed board by soldering before the printed board is guided towards the element, and positioning the printed board on the element with the help of the at least one guide element. Thus a difficult manual or automatic insertion of separate guide pins in the element, as mentioned in the xe2x80x98Description of related artxe2x80x99, is avoided.
Suitably, the method further comprises the steps of: positioning at least one cover on top of the printed board with the help of the at least one guide element; and fastening the printed board and the at least one cover to the element by at least one fastener, for example at least one screw that is screwed into to each other corresponding holes in the at least one cover, the printed board and the element. Hereby is achieved that the guide element also is used for positioning a cover, which preferably protects the essential parts of the printed board.
Preferably, the method further comprises the steps of: mounting at least one electronic component on the printed board before the printed board is guided towards the element, and fastening the at least one electronic component to the printed board by soldering. Hereby is achieved that the electronic component and the guide element suitably are placed during the same step and also soldered to the printed board during a common step.
Advantageously, the at least one guide element and the at least one electronic component are mounted by an automatic apparatus, such as a so-called pick and place machine, and the at least one electronic component is surface-mounted.
If needed, the method comprises the steps of: mounting at least one mechanical component on the printed board before the printed board is guided towards the element and fastening the at least one mechanical component to the printed board by soldering. The at least one mechanical component is mounted by an automatic apparatus, such as a so-called pick and place machine, and the at least one mechanical component is surface mounted. Hereby is achieved that electronic components, guide elements, and mechanical components, such as shields for electromagnetism, suitably are placed during the same step and may be soldered to the printed board during a common step.
Suitably the at least one guide element is mounted to the printed board by being inserted into a corresponding through-hole in the printed board. This allows larger limits of tolerance than the press fit between a guide pin and an element as mentioned above, since capillary effects from solder paste during the soldering encourage a centering of the guide element in the through-hole.
The invention also provides a guide element for the method according to the invention. The guide element comprises at least one solder surface, which preferably is flat, adapted to be soldered to a printed board in order to fasten the guide element to the printed board by soldering, and at least one guide surface adapted to guide the printed board into a position in relation to an element, to which the printed board is to be fastened. Preferably the at least one guide surface is cylindrical in order to fit a corresponding through-hole in the printed board and the element.
To facilitate insertion of the guide element in a corresponding hole in the element, the guide element comprises an end section with a conical or frusto-conical shape.
Suitably, the guide element comprises at least one flange that directs and positions the guide element in a desired direction and desired position relative to the printed board when the guide element is placed in a hole in the printed board. Moreover, the flange may be substantially perpendicular to a longitudinal axis of the guide element and preferably comprises the at least one solder surface. Furthermore the flange may be positioned substantially in the centre of the guide element in the longitudinal direction of said guide element.
In order to guide, for example, a cover placed over the printed board and facilitate insertion of the guide element in the cover, the guide element comprises a second end section with a substantially conical or frusto-conical shape.
If required, at least one of the end sections comprises at least one slot which divide the end section into axially extending fingers that are deflectable substantially radially inwards.